A hydraulic motor is provided with a diagonal plate received in a motor casing, a cylinder block with an output shaft penetrating through the diagonal plate and a plurality of pistons that reciprocate inside the cylinder block. A tip of the plurality of the pistons contacts the diagonal plate. The plurality of the pistons expand caused by hydraulic pressure applied thereon. The plurality of the pistons expand in turn, so that the cylinder block rotates around the output shaft.
A hydraulic motor described in a Japanese Unexamined Patent Publication No. 8-61212 published in 1996 by Japan Patent Office is provided with a brake disc and a brake driving device. The brake disc rotates together with a cylinder block. A rotation of an output shaft ceases by braking a rotation of the brake disc with friction resistance caused by pressing the brake disc on a motor casing with the brake driving device.
In the above-mentioned earlier hydraulic motor, however, a space in the motor casing is divided into two chambers by the brake disc. Accordingly, when the hydraulic motor operates, a pressure difference between the two chambers is likely to arise, which possibly presses the brake disc on the motor casing even if the braking by the brake driving device is released.
Therefore, a braking force exerts on the output shaft during operating the hydraulic motor and an efficiency of the hydraulic motor threatens to deteriorate.
Accordingly, in the related art an attempt to cancel the pressure difference between the two chambers due to disposing a communicating bore at the brake disc is made. However, flow of an operating fluid is interrupted by rotation of the brake disc and as a result, the pressure difference between the two chambers still occurs. The brake disc is pressed on the motor casing by reason of the pressure difference occurred and as a result, a friction resistance occurs at the brake disc. Namely, when the hydraulic motor is operating, the friction resistance occurs at the brake disc.